ExTrA

Exoplanets in Transits and their Atmospheres — Searching for nearby exo-Earths around cool stars

Amongst a new generation of planet-hunting telescopes, the French national ExTrA project is sited at ESO’s La Silla Observatory in northern Chile. Comprising three 60-centimetre telescopes, the facility utilises the transit method to detect Earth-sized worlds in the Milky Way — looking for slight dips in the light from a star that could be caused by a planet passing across the star’s disc and obscuring some of its light.

ExTrA’s mission is to detect Earth-sized worlds around stars of a kind known as M dwarfs. These stars were selected because of their small size, which makes it easier to detect orbiting planets via the transit method; a transiting planet will block a greater fraction of the light from a smaller star. Focusing on small, nearby M dwarfs therefore maximises the chances of making discoveries with the telescope.

ExTrA employs a novel approach to the transit method; it observes at infrared wavelengths, where M dwarfs are brighter, and adds spectral information to the usual photometric measurements. Its three telescopes collect light from the target and from four comparison stars; the light is then fed through optical fibres into a near-infrared spectrograph. In this cost-efficient approach, the light from all three telescopes is fed into just one multi-object spectrograph.

This innovative approach of adding spectroscopic information to traditional photometry helps to correct the disruptive effect of Earth’s atmosphere, as well as errors from instruments and detectors. In addition, M dwarfs appear much brighter at infrared wavelengths — increasing the precision achievable and boosting the survey’s efficiency.

Science goals

The facility will attempt to study the structure and composition of Earth-sized worlds, and address some fundamental questions about planets in our galaxy. These could include how common planets like this are, the dynamics of multi-planet systems, and the sorts of environments that lead to their formation.